稳定大气环境边界层微波折射率结构常数建模

大气湍流对电波传播有着重要影响.结合M-O近地层相似性原理以及Tatarski和Bulk方法估算方法推导出一种计算温度结构函数,湿度结构函数和混掺结构函数的新方法.运用适当的参数建立了一种利用常规气象要素估算稳定大气条件下微波波段折射率结构常数的模型.该模型只需要测量下垫面温湿度及所测量处高度的温度、湿度、风速就能估算出当前大气环境中的折射率结构常数.分析模型可以知道:湿度对微波波段折射率结构常数影响较大,风速变化无明显影响.混掺结构函数和湿度结构函数所占的作用比较明显,在计算时不可忽略.该模型为研究微波在湍流环境中的传播提供了理论参考.     

高速铁路列车风对折射率结构常数的影响

大气湍流在时间和空间上的运动对无线电波有着重要影响.用折射率结构常数描述湍流是目前最主要的方法.本文通过建立折射率结构常数模型,来研究大气湍流对高速铁路的无线通信的影响.通过运用空气动力学基本原理,引入列车风并推导出一种适用于高速铁路下的折射率结构常数模型.该模型输入的主要参数有列车风、温度和湿度.通过理论推导和仿真分析得出了列车风对折射率结构常数的影响主要有列车边界层的厚度、距列车侧壁的距离、列车高度和列车车速.分析模型可以知道,车速、温度、湿度对其影响较大,其次是列车边界层的厚度、距列车侧壁的距离,列车高度.并为研究高速铁路下的电波的传播提供了理论参考.     

40GHz毫米波室内传播损耗分析

介绍了室内毫米波的传播特性,通过改变发射天线的位置实际测量了37.598GHz和40GHz毫米波在实验室内和走廊的接收功率,并对数据进行处理,得出毫米波室内视距条件下的基于衰减因子损耗模型的传播路径损耗因子n。分析两种频率的毫米波测量结果,对于室内基站位置的选择和无线终端的设计以及40GHzmm-ROF系统的室内覆盖提供依据。     

全光纤电流互感器传感机理建模分析

该文针对全光纤电流互感器测量精度受各种内外部因素影响的问题,建立全光纤电流互感器微元传感单元的分布参数模型,从本质上解释传感单元中线性双折射的产生机理,即极化率张量对角元不相等的作用结果;并结合光电转化的数学模型得到全光纤电流互感器的开环机理,提出提高全光纤电流互感器(fiber-optical current transformer,FOCT)测量精度的方法:采用新型传感材料或新型传感头结构及引入反馈信号构建闭环结构。采用COMSOL有限元数值分析方法,实现光场和磁场的耦合。分析双折射、被测电流、纤芯折射率、光纤的弯曲半径对测量结果的影响。研究表明,线性双折射是由传感材料的折射率变化引起,会降低测量灵敏度;同一双折射,在不同的外界条件(如被测电流)下,对测量结果的影响不同;对于不同的纤芯折射率,折射率越小,传输相同距离后,旋转角越大;微元传感单元中由弯曲半径引起的线性双折射较小,通常可采用分段补偿法,得到理想的Faraday旋转角;最后通过仿真分析与现有经典公式的比较,验证分布参数模型的有效性,为后续FOCT传感单元中的光场、磁场、温度场、应力场等复杂的多物理场耦合提供模型基础。     

变压器保护两种转角方式的比较

对变压器保护中的△→Y和Y→△两种补偿方式进行了研究.通过理论推导得出了两种补偿方式的算法公式.针对两种转角方法,在变压器保护中对于涌流的识别是否可以采用按相闭锁(单相闭锁单相)方案,用EMTP建模进行了大量的仿真分析.理论和仿真分析的结果证明了两种转角方式在涌流识别中的相似性.     

混合路径土壤对雷电辐射磁场的影响分析

为提高雷电监测的定位和幅值估算的精度,实现对雷电本体的遥测,研究了雷电辐射磁场沿混合路径土壤传播的规律。给出了单层土壤、两层土壤、三层土壤和任意土壤构成的混合路径上方的雷电辐射磁场频域计算公式,进而得出了混合路径情况下雷电辐射磁场的时域计算方法。分别以两层土壤和三层土壤构成的混合路径为基础分析了土壤电导率对雷电辐射磁场的影响,得出其影响主要体现在雷电辐射磁场的上升时间的变化上。通过分析任意土壤构成的混合路径情况下衰减函数计算结果的正确性,建议对于此类模型全部采用基于Millington经验公式的简单方法进行分析。最后总结了考虑混合路径情况下依据雷电磁场观测波形反演雷电流本体的方法。     

舰船间水平间距及舷偏角测量

针对海上舰船补给活动中船间相对位姿监测的需求,详细介绍了船间水平间距及舷偏角的测量。采用非接触式光电跟踪瞄准结合激光测距方法,对目标进行识别测量,结合船自身姿态测量获得最终测量结果。根据测量原理,建立了动平台下的计算模型。采用四元数的方法进行了公式推导,应用蒙特卡洛法对测量公式进行数据仿真,并对测量误差进行了估算。仿真结果表明水平间距测量精度优于15 mm,舷偏角精度优于1',通过实验室实际测量,验证了这一结果。     

太赫兹波在绝缘材料测厚中的应用

太赫兹波在绝缘介质中的传播速度与折射率相关,在材料折射率已知的情况下,通过太赫兹波的时延特性可实现材料的厚度测量。电网设备中的许多绝缘部件由于结构不规则、不能破坏,其厚度难以用常规方法直接测量,而太赫兹波的特性使其具有一定的适用性。文中结合传播模型分析了太赫兹波传播过程中多次反射波的叠加效应,测试了不同厚度的硅橡胶、环氧玻纤平板材料的透射波与反射波特性,分析了太赫兹时域波形在介质中传播的时延、衰减与介质厚度的关系,提出了在透射和反射两种模式下绝缘材料厚度测量的方法。研究表明,太赫兹透波在绝缘材料的厚度测量中有较高的精度,可应用在材料测厚、护套均匀性测试等场合。     

基于抛物方程的湍流散射研究

在远距离的电磁信号传输过程中,大气散射会对微波传输产生不可忽视的影响。为了定量分析这种影响,从大气湍流的结构出发,在同时考虑湿度以及温度的基础上得到电波波段的湍流大气结构常数,然后基于统计随机理论建立了湍流大气散射模型,采用抛物方程模拟并分析了大气湍流对微波传输的影响。仿真结果表明,大气湍流引起的散射效应在超视距区域才趋于明显;随着传播距离的增加,沿传播方向的场强幅度出现随机的起伏;大气散射效应会随着高度的增加而减弱。     

牙模激光三维扫描仪结构与标定方法研究

针对口腔医学需求,设计了一种牙颌模型三维激光扫描仪的机械结构,分析了影响测量精度的因素,给出了坐标转换公式,讨论了仪器的调整与标定方法,介绍了一个采用该结构的实际装置及实测结果.结论:采用该方案的激光三维扫描仪具有较高的精度,工作可靠,扫描盲区少,可用于修复体CAD/CAM系统光学印模的制取.     

Novel multimode fiber for narrow-band Bragg gratings

We propose a novel multimode fiber structure with modal propagation characteristics tailored to facilitate the creation of narrow-band high-reflectivity fiber Bragg gratings. The fiber structure proposed consists of concentric cylindrical shells of higher and lower refractive index material. A full vector second-order finite-element method is used to analyze the proposed multimode fiber structure. Simulations of the modal profiles show that high-order modes are localized to particular high-refractive index shells. We present the theoretical characterization of the modal propagation constant as a function of inner shell radius, shell separation, and harmonic-mode parameter. It is shown that a fiber with a minimum inner shell radius of at least 25λ (where λ is the vacuum wavelength), and a minimum shell separation of at least 10λ provides a reasonable tradeoff between fiber size and grating performance. A simulation of the multimode fiber grating shows that a grating with a full-width at half-maximum bandwidth on the order of 10^-4λ is theoretically possible, if optical power is launched strictly into modes with angular harmonic parameter p=1     

隧道中反射和绕射对毫米波传输特性影响的研究

主要研究和探讨了毫米波在圆形截面的弯曲隧道中,由于反射和绕射的存在,对其传输特性的影响;然后针对在直隧道中,没有绕射存在的情况,将此时的传输特性进行了对比。本文的研究方法主要是将高斯波束的谱特性、射线跟踪法(RT)和一致性劈绕射理论(UTD)结合起来,得到了接收功率、时延扩展和路径损耗等传输特性参数的理论模型。运用MATLAB进行仿真,比较了不同隧道曲率半径和不同频率下的路径损耗,以及在同一频率不同隧道曲率半径下和同一隧道曲率半径和不同频率下的时延扩展。最后,针对这些传输特性得出了相应的结论,为毫米波的隧道研究奠定了一定的理论基础。     

大气折射对跟踪天线指向的影响及修正方法

本文对对流层大气折射造成的跟踪天线指向误差进行研究。采用三段式指数型模型模拟对流层大气折射率分布状态。对电磁波折射传播路径进行微分建模，区别于传统的近似计算，给出基于地面站和飞机海拔高度以及两者间水平距离的折射误差精确计算公式。最后对跟踪天线仰角误差和飞机天线俯角误差进行修正，给出修正方程，并提出一种改进方法，将误差修正方程进行扩展，用于地面跟踪天线处于负仰角情况下的误差修正问题。利用MATLAB计算在标准大气条件下的误差修正问题，验证上述方法的合理胜和可行性。     

New Estimation Method of Millimeter‐Wave Power by Dealing Optical Power

This report is regarding to a new measurement of a millimeter‐wave using optical power. A calorie meter conventionally has been used to measure the millimeter‐wave power above 100 GHz, which has high temperature dependence, and therefore finds the difficulty of small power measurement. To overcome this difficulty, we propose the new method to estimate millimeter‐wave power by measuring optical power of optical 2‐tone signals. This proposed method has low temperature dependence due to optical power, and therefore makes the small power of the above millimeter‐wave enable. The principle and the estimation methods of this new method, and its experimental methods and results are explained. Noble results show that the difference between estimated and experiment value is maximally 0.99 dB at 110 GHz, the measurement possibility of frequency extending more than 170 GHz is referred to.     

炉内双排换热器管阵列声传播特性的数值研究

根据炉内单排换热器管阵列声传播特性的研究,利用Twersky声散射理论和光学法布里-珀罗干涉原理,研究双排换热器管阵列的声传播特性,分析了双排管阵列对声波的反射、透射和衍射机制,给出了计算炉内双排管阵列声透射与反射系数的数学模型,计算了炉内双排管阵列的声反射与透射系数随声波频率、管阵列横向与纵向节距等参数变化的关系曲线,以及传输泄漏声辐射信号时的聋带和聪带的分布特征,指出双排管阵列对声波的截止带和导通带的位置与管阵列结构参数和炉内温度有关,并在炉内实际环境参数下对声传播特性进行了数值计算,为研究多排管阵列的声传播特性提供了方法,也为利用管阵列透射声谱判断管道泄漏口发生在管排阵列中的位置提供了依据。     

Transmission line tower models in frequency domain

This paper proposes a transmission line tower model in frequency domain for back-flashover analysis, which has a uniform characteristic impedance and a uniform propagation constant. Most conventional tower models are constructed as a constant and uniform parameter transmission line that is independent of frequency. However, the authors' models have the frequency dependency of the characteristic impedance and the propagation constant to express the frequency characteristic of the transmission line tower. Two models, a cylinder model and a four-frames model, were investigated to simulate the tower. Their surge responses were computed by numerical electromagnetic field analysis based on the moment method. From the results, the two-port circuit constants were calculated in the frequency domain, and the characteristic impedance and the propagation constant are derived from them. The complex artificial oscillations appeared in frequency dependency of the parameters because the parameters were calculated numerically. The equations with analytical form were obtained by approximating with a smooth curve to the oscillations. It is possible for the equations to represent the transmission line constants as a function of geometrical parameters such as the tower height and the radius. Close agreement was found between the surge response computed by the numerical electromagnetic field analysis and the result calculated by inverse Laplace transformation of the analytical equations. To prove the model, the experimental results were compared with the computed ones of the cylinder model that has arms, and it was shown that the new model agrees closer with the experiment than conventional transmission line models.     

目标跟踪在坦克防撞中的研究与应用

考虑到毫米波雷达在噪声源较多的战场上存在较大的测量误差,为了提高毫米波雷达在坦克防撞系统中采集数据的可靠性,提出了用交互式多模型卡尔曼滤波算法对坦克前方机动车辆进行准确的目标跟踪。该算法运用不同机动模型的卡尔曼滤波器进行并行处理,以模型匹配似然函数为基础更新模型概率,并组合所有滤波器的修正后的状态估计值以得到状态估计。仿真结果表明,该算法能够有效的跟踪坦克前方行驶的车辆,获悉其距离、速度等信息,具有跟踪精度高的特点,降低了虚警率。     

Theoretical characterization of multilayer photonic crystals having a 2D periodicity

We present a new, very accurate and fast model of photonic bandgap (PBG) structure characterized by a two‐dimensional (2D) periodic change of the refractive index and finite height, therefore named quasi 3D PBG. The new model is based on the Floquet–Bloch formalism and allows to find all the propagation characteristics including the space harmonics and the total field distribution, the propagation constants, the guided and radiated power and modal loss induced by the 2D grating. A clear explanation of the physical phenomena occurring when a wave propagates inside the 2D periodic structure is presented, including the photonic bandgap formation and the radiation effects. The approach does not require any theoretical approximation, and can be applied to study rigorously any PBG‐based multilayer structures. We have applied the model to investigate several structures for both optical and microwave applications. Copyright © 2005 John Wiley & Sons, Ltd.